Making dental restorations is important in many situations. First, human teeth are subject to naturally occurring breakdowns such as decay and wear. Decay will normally be corrected through semi-permanent means such as fillings and the like. However, after numerous years, decay can reach a point where restoration of the tooth through an inlay, onlay or crown becomes necessary.
Another situation in which a dental restoration is important is where an accident involving a blow to the mouth results in chips, cracks and/or breaks of the tooth. In these situations, the patient not only requires relief from the discomfort associated with the dental injury but also desires to have the injured tooth or teeth returned to their pre-accident appearance. Once again, a restoration in the form of an inlay, onlay or crown is necessary.
Using conventional means of preparing restorations is often time consuming for both the dentist and patient. Generally, more than one visit by the patient is necessary. The first visit entails preparing the tooth or teeth for restoration and fitting the prepared tooth/teeth with a temporary substitute until delivery of the final restoration. During this visit, final impression is taken and a working model is prepared with individual teeth separated and indexed to be able to be precisely reassembled. The model teeth are carefully trimmed to indicate the margins of the final restoration. This trimming preferably is done by the dentist. The model is then used by a dental lab technician who is usually located in a remote dental laboratory to fabricate the final restoration. This procedure usually takes two to three weeks to be finished, at least in part due to the time involved in firing and casting porcelain. Porcelain is a material commonly used in making dental restorations. There are multiple steps and each step introduced inaccuracies.
Once the dentist receives the final restoration back in the office from the laboratory, the patient returns for another appointment. This appointment involves removing the temporary substitute and then adjusting and permanent placement of the final restoration. During this appointment final adjustments to accommodate chewing motions of opposing teeth are performed by the dentist in the patient's mouth and then the restoration has to be polished again.
Communication with a lab technician at a remote lab frequently is imperfect and the restoration received is not exactly what was expected. This requires repeating certain steps which were already taken resulting in frustration for both dentist and patient.
Furthermore, certain procedures, such as fitting a crown under a partial denture clasp that already exists is extremely inaccurate using any existing method. In fitting a crown under a partial denture clasp, the dentist is required to spend a considerable amount of time in contouring and fitting the restoration in regard to the other surrounding and opposing teeth. This problem is resolved with the method being presented. Besides the time involved, when a crown does not exactly fit the existing partial denture clasp, the partial denture will not seat properly and this in turn will disrupt the patient's occlusion and lead to further pathology.
Recently, computer reconstruction of dental restorations has become commercially feasible. Generally, these systems involve an optical reader which generates a computer-read signal. Siemens.RTM. and Sopha.RTM. CAD/CAM both have commercially available systems which optically read tooth areas in conjunction with dental reconstruction. Sopha.RTM. uses the optical impression system as disclosed in U.S. Pat. No. 4,611,288 to Duret et. al.
The use of optical impression systems greatly reduces the amount of time involved in preparing a restoration as compared with conventional methods, but still has several drawbacks.
The Siemens.RTM. reconstruction system eliminates the requirement of a model by using optical impressions made by a specially designed camera. The camera is used in the mouth to read the contour of the tooth. Using the camera in the mouth results in a certain amount of inaccuracy due to moisture in the mouth and on the tooth surface, use of an imaging powder over the prepared tooth, the presence of the tongue and difficulty in manipulating the camera in areas further back in the mouth. In addition, this method does not record the opposing tooth and cannot fit a restoration to a partial denture clasp that already exists. As a result, the restoration may require extensive contouring by the dentist before the restoration is properly fitted. As previously discussed, contouring is a time consuming step.
The Sopha Bioconcept.RTM. CAD/CAM is different from the Siemens.RTM. system in that it requires a working model before the restoration is carried out. This added step increases the time to make and the expense of the final restoration. In addition, the Sopha Bioconcept.RTM. system does not record the opposing teeth. Rather, this system fabricates a tooth with a textbook morphology and the dentist makes modifications on a computer screen and the model and in the mouth to customize the final design. The Sopha Bioconcept.RTM. cannot fit a crown to a partial denture clasps that already exists.
Like the Sopha.RTM. system, U.S. Pat. No. 4,324,546 to Heitlinger et. al. also requires a model. This patent describes a method of manufacturing dentures, wherein a prepared tooth stump is reproduced as a working model. The reproduction of the tooth stump is accomplished electro-optically to produce signals which are fed through a computer which in turn operates a milling machine. This, like the Sopha.RTM. method, entails the use of a working model.